The present invention relates to a method and apparatus for the indirect heating of a process gas within a reaction chamber for an endothermic reaction, particularly, a catalytic reaction wherein the wall of the reaction chamber is surrounded by a flow of hot smoke gas for heating the interior of the chamber and particularly the process gas within said chamber.
A particularly preferred field of use of the present invention is the catalytic vapor reforming of hydrocarbons for manufacturing a gas that is rich in hydrogen. The process gas envisioned here is a blend of water vapor or steam, some hydrocarbon and/or CO.sub.2. This blend is fed to a reaction chamber at a temperature of from 400.degree. to 600.degree. C. with pressures of up to 4 MPa. The reaction in the general sense may be performed through a plurality of catalyst filled tubes or pipes and the surrounding hot gas, so that the gas in the chamber is heated to 750.degree.-900.degree. C. With the assistance of the catalyst, the process gas then reacts endothermically to yield a hydrogen rich gas containing amounts of CO, CO.sub.2 and an excess of steam and residual hydrocarbons. The hydrogen content of the product gas depends on the excess of the water vapor or steam, and on the temperature and pressure in the catalyst filled tubing. An increase in excess steam and/or temperature in the chamber will result in a corresponding increase in yield of hydrogen while an increase in the pressure causes a drop in the hydrogen yield.
European patent 194067 B1 describes an apparatus for carrying out such an endothermic catalytic reaction of a process gas. The apparatus includes a casing or housing with several, parallely arranged, catalyst filled tubes of the blind bore variety. Means are provided in the casing for feeding and discharging smoke gas so as to surround and heat the outer surfaces of these tubes while the process gas flows through this tubing while taking up heat. The catalyst filled tubes are surrounded by a tubular jacket which together with the tube forms a ring shaped chamber and a cylindrical gap. The jacket covers most of the respective catalyst filled tubing. The feeding of the smoke gas results in directly hitting the blind end of the catalyst filled tubes. This direct onflow and the immediate surrounding flow of the hot gas was found to be disadvantageous because in fact it establishes a very high thermal load on the blind end of the catalyst filled tubing. In certain critical spots the temperature of the jacket is actually raised to such a degree that it was damaged fairly soon and had to be exchanged rather frequently. This detriment can be counteracted by cooling the smoke gas which on the other hand is a somewhat wasteful procedure. One may also provide a heat exchanger in front of the tubing as far as the smoke gas is concerned or one can recycle some cooled smoke gas in order to reduce the temperature without any loss in thermal energy as far as the oncoming flow is concerned. A heat exchanger may also be used here but it was found that all these aspects provide for a rather expensive way of dealing with the situation.
Reducing the thermal load of the catalyst filled chamber is known by means of providing it on the outside with a ceramic jacket particularly in the area of the tubing which is hit first by the hot flow of smoke gas. However it was found that this kind of ceramic protective jacket is prone to respond rather drastically to temperature variations by expansion and contraction resulting in a damage to the ceramic and sooner or later in damage to the catalyst filled tube itself.